The invention relates to a method for determining the fat or oil content of a sample.
The invention further relates to an apparatus for determining the fat or oil content of a sample.
In order to be able to measure the fat or oil content of samples, in particular of foodstuffs, in a quick way, it is known to examine the sample by means of low-resolving, pulsed nuclear magnetic resonance (NMR), for example with a resolution of 10−5 in a constant magnetic field of 1 tesla field strength, something which corresponds to a nuclear magnetic resonant frequency of protons of the order of magnitude of 50 MHz. Specially designed NMR spectrometers are commercially available for this purpose, for example from the applicant under the type designation of “minispec”.
In the case of samples which also have a considerable water content in addition to the fat or oil, such a measurement is, however, attended by the problem that the strong proton signal of the water disturbs the measurement of the fat or oil content. It has therefore already been proposed in DE 41 33 642 C1 to predry the sample in an oven, for example a microwave oven, or to remove the water content at least partially by means of chemical drying.
U.S. Pat. No. 6,548,303 B2 describes a method and an apparatus of the type already mentioned, in the case of which method and apparatus the sample is relieved of its water constituent by predrying in a microwave oven. The sample is firstly introduced into a separate microwave oven for this purpose. There, the sample is heated in a controlled fashion under the action of a microwave field with a frequency of 2.45 GHz and while being monitored by a sensor, and is thereby dried. The sample is located in this case on a balance arranged in the microwave oven such that the weight loss of the sample is measured as drying proceeds, and it is therefore also possible in addition to determine the water content. After termination of the drying operation, the sample is removed manually from the microwave oven and transferred into a separate NMR analyser in which the nuclear magnetic resonance measurement then takes place.
This known mode of procedure has the disadvantage that the sample must be handled in a complicated fashion. In order to enable this handling, the substance of the sample must firstly be applied to a tissue fragment of quartz or glass fibres which acts as a substrate. This tissue fragment is transparent to microwaves and free from proton signals in the NMR measuring range of interest here. Since, because of the fat or oil fractions, the sample material liquefies as it dries out, at least for the NMR measurement the sample substance applied to the tissue fragment must further be wrapped in a film which consists of polytetrafluoroethylene (PTFE). All these steps are clearly very complicated and time-consuming. Automated measurements on a multiplicity of samples are therefore impossible with the aid of this known mode of procedure.